This invention is applicable to the art of multiturn stator windings of high voltage hydroelectric generators and more particularly to a new and improved turn insulation system having a higher dielectric strength insulation on one of the outer strands of the turn and a lower dielectric strength insulation on all remaining strands. This invention is also applicable to turbine generators and motors where multiturn stator coils are utilized.
The multiturn stator coils of many high voltage dynamoelectric machines contain two or more identical turns, each turn being constructed of several individually insulated copper strands. These strands are usually individually insulated with a low dielectric strength insulation designed to adequately protect them from the low strand to strand voltages generated during the operation of the dynamoelectric machines. This strand insulation usually consists of one or two layers of bonded DACRON-glass tape, fused DACRON-glass filament or, rarely, enamel. The insulation thickness of these several strand insulation types varies from approximately 0.003 inches for the lower grade enamel insulation to approximately 0.011 inches for the higher grade bonded DACRON-glass tape insulation. The dielectric strength of all these strand insulation types increases almost linearly with the thickness of their insulation but their effectiveness is greatly influenced by the mechanical and thermal operating conditions of the machine in which they are installed. In practice, the selection of the specific type of strand insulation to be used depends equally on dielectric strength considerations as on cost and availability considerations.
The design requirements of high voltage generator and motor multiturn stator windings result in much higher operating voltages occurring between adjacent turns in each coil than between individual strands in each turn. In addition, it is known that voltage surges or steep front impulses occur during the operation of high voltage machines, which surges and impulses also result in high voltages occurring between the turns of multiturn coils. Such relatively high operating voltages combined with high voltage surges and impulses can and do result in premature turn to turn insulation failure. U.S. Pat. No. 2,201,845, May 21, 1940 describes a method of reducing the effects of such high turn to turn voltages by placing coils with high turn to turn insulation near the high voltage end of the electric parallel whereas coils with lower turn to turn insulation are placed near the low voltage end of the electric parallel. Of some interest is also U.S. Pat. No. 4,724,600, Feb. 16, 1988. Another invention which addresses the difficulties associated with manufacturing of the various state of the art strand and turn insulation systems is U.S. Pat. No. 5,099,159, Mar. 24, 1992. All the above inventions, however, describe manufacturing and installation methods and materials which are relatively difficult and expensive.
Most present day manufacturers of high voltage multiturn stator coils utilize lower dielectric strength strand insulation systems and rely on the additional application of one or more dedicated taped layers of relatively higher dielectric strength material such as mica, applied around each turn of the coil, to protect the coils from the high turn to turn voltages. The lower dielectric strength strand insulation in these types of multiturn coils usually consists of one or two taped layers of DACRON-glass bonded directly to the copper surface of the wire or a similar thickness of DACRON-glass filament fused over enameled wire. More rarely, usually in lower voltage machines, plain enameled wire without the benefit of additional layers of DACRON-glass is used. Alternatively, some manufacturers utilize exclusively higher dielectric strength mica insulation, applied directly to the wire strands, which mica insulation protects the coil from both the higher turn to turn voltages as well as from the lower strand to strand voltages. Because mica is much more expensive than DACRON-glass, however, this type of all-mica insulation system is considerably more expensive to manufacture. In addition, because these all-mica insulation systems do not normally utilize any additional dedicated turn insulation, the mica strand insulation is considerably thicker than the DACRON-glass type of strand insulation and, as a result, the copper strands become smaller in cross-section.